Attribution of Hydrologic Changes in a Tropical River Basin to Rainfall Variability and Land-Use Change: Case Study from India

Abstract

In recent decades, several parts of the world have been facing severe droughts and frequent floods against a background of anthropogenic influences and changes in climatic variables. For efficient management and adaptation measures, it is important to understand the relative effect of climate variability and anthropogenic influences. The Nagavalli River Basin (NRB), located in India, has experienced significant changes in hydroclimatic variables and land use in the last decade and therefore serves an ideal basin for investigating the attribution of rainfall variability and land-use changes. In this study, the characteristics of precipitation, temperature, and streamflow were analyzed for the period 1970–2012. It was observed that there is a significant increasing trend in precipitation and streamflow. Further, there have been substantial land-use change in terms of scrubland conversion. Investigations on rainfall-runoff coherency using wavelet coherence showed that there were noteworthy changes during the periods of 1991–2001 and 2002–2012. The contributions of land-use and rainfall variability to the changes in the streamflow were quantified using a semidistributed hydrological model (Soil Water Assessment Tool). The results showed that for the whole NRB, the variations of mean annual streamflow in 2002–2012 were primarily affected by rainfall variability with reference to 1990s, whereas human activities played a complementary role. The quantitative assessment revealed that rainfall variability resulted in an increase in runoff by 103 mm in 2002–2012 for the whole catchment, accounting for 41.52% of runoff changes relative to the 1990s. However, land-use changes are responsible for a decrease in runoff by 59 mm during the period of 2002–2012, which accounts for −23.54% of runoff changes. Overall, it was observed that the agricultural intensification in terms of scrubland conversion counteracted the effect of rainfall variability, resulting in the reduction in the margin of increase in streamflow.